Genetic variation and combining abilities for vigour and yield in a recurrent selection programme for cacao

The low genetic diversity of cacao germplasm collections in West Africa is often cited as a limitation to further yield improvement of the crop in its major production countries. Twelve clones obtained from first cycle selection for yield and clones available in international genebanks were tested for their combining ability for key agronomic traits by using these as males in crosses with five female clones available in the Seed Gardens in Ghana. Progenies obtained from a North Carolina II (NC II) mating design and standard varieties were planted at two sites that contrast in terms of soil fertility, rainfall total and distribution. Fifty-eight progenies obtained from the 5 x 12 incomplete NC II mating design and six other progenies, of which two were standard varieties, were evaluated at the more favourable location. At the less favourable location, 44 progenies of the NC II mating design and 20 other progenies, including the two standard varieties, were planted. A randomized complete block design with four replications was used at both locations. Progenies with PA 7 as female parent had better seedling survival at the location with poorer production conditions. Low levels of genetic diversity among female clones at 86 single nucleotide polymorphism loci was reflected in larger standard errors for the additive variance due to female parents relative to that due to male parents. Non-additive genetic effects were much larger than the corresponding additive variance components for all traits at each of the two locations. Average yields over the fourth and fifth years after planting ranged from 023 to 129 t/ha/year. The standard varieties were not among the best progenies for seedling vigour, yield or yield efficiency (yield per unit trunk cross-sectional area (TCSA) per year). Narrow-sense heritability estimates were higher for three estimates of tree vigour (estimated as increase in TCSA) than for yield traits. Heritability for yield efficiency was a moderate of h(2) = 05 +/- 023. The study indicates that further yield increments are attainable by selecting and crossing among the early cacao introductions into West Africa.